Structural design of bending-resistant all-solid fiber with large mode field

  Objective   Compared with traditional solid-state lasers and gas lasers, high-power lasers have a series of advantages such as high stability, flexibility, good beam quality, and energy concentration. In recent years, the output power of fiber lasers has increased to 10 kW. There are important applications in mechanical, medical, communication, sensing, and other fields. However, fiber lasers are usually limited by nonlinear effects such as stimulated Brillouin scattering, stimulated Raman scattering, and four-wave mixing with increasing output power. The massive intensity of the optical field inside the high-power fiber laser usually causes specific damage to the fiber core. Traditionally, the method to avoid core burning is to enlarge the effective mode field area by increasing the fiber diameter. However, it leads to an increase in the output modes and generates mode competition to compromise both the output quality of the beam and bending resistance. Therefore, it is necessary for fibers to achieve a large mode field area with the single-mode operation. For the purpose, a novel fiber with a large mode field area, low bending loss and symmetric is designed in this paper.

  Methods   A novel fiber with a large mode field area, low bending loss and symmetric structure is designed in this paper. The proposed fiber consists of a trapezoidal refractive index ring in the core and a multi-trench in the cladding (Fig.1). COMSOL Multiphysics commercial software based on the full vector finite element method is chosen to study the bending properties of the designed fiber. Mapped mesh and free triangle mesh are used to mesh the proposed structure (Fig.2). The bending loss and single-mode operation are used to evaluate the bending properties. The numerical simulation was carried out by changing the fiber related structure, and the optimal structure is verified by thermal load.

  Results and Discussions   The bending loss and electric field mode distribution of trapezoidal refractive index ring, rectangular refractive index ring and triangular refractive index ring are compared and analyzed. The experimental results show that trapezoidal refractive index ring has more advantages (Tab.1, Fig.7). The structure of multi-trench in the cladding limits the mode field in the core of fiber. When the number of trenches is greater than 2, the mode field area basically remains the same (Fig.8, Fig.9). The results show that when the wavelength is 1 550 nm and the bending radius is 20 cm, the bending loss of fundamental mode is only 0.056 868 dB/m, while that of high order modes is 3.58 dB/m. The mode field area is 2 313.67 μm², which meets the requirements of high-power fiber laser (Fig.6). As the thermal load increases, the bending loss of fundamental mode, high order modes and effective mode field area all decrease. When Q is 9.5 W/m, the bending loss of high order modes is less than 1 dB/m, at which time the fiber cannot achieve the single-mode operation (Fig.10).

  Conclusions   A novel bending-resistant fiber with large mode field area is proposed. The effects of different structural parameters on bending properties and mode properties are analyzed by the full vector finite element method. The trapezoidal refractive index ring as a resonant ring can fully couple with modes and filter out high order modes, which is beneficial to obtain a larger mode field area. The increase in the number of trenches in the cladding enhances the effective refractive index difference between the core and the cladding, which reduces proposed fiber bending loss. The results show that at a wavelength of 1 550 nm and a bending radius of 20 cm, the bending loss of fundamental mode is only 0.056 868 dB/m and the bending loss of high order modes is 3.58 dB/m, with a loss ratio of 63 and a mode field area of 2 313.67 μm² for single-mode operation. The effects of different thermal loads on fundamental mode, high order modes and effective mode field area are analyzed. When the thermal load Q is less than 9.5 W/m, proposed fiber can achieve a stable single-mode operation. The fiber is insensitive to bending and has a broad development prospect in the field of optical communication devices such as high-power fiber laser amplifiers.